Resistance welding a porous metal layer to a metal substrate utilizing an intermediate element

ABSTRACT

One aspect of the present disclosure provides a method of manufacturing an orthopedic prosthesis. This particular method includes providing a porous metal layer ( 22 ) positioned against a metal substrate at an interface between the porous metal layer and the metal substrate. Additional steps include providing an electrode ( 132 ) and providing an intermediate element or coating positioned between and in contact with the electrode and the porous metal layer where the intermediate element includes tantalum in contact with tantalum of the porous metal layer. In a further step, an electrical current is directed to the interface between the porous metal layer and the metal substrate to bond the porous metal layer to the metal substrate.

CLAIM OF PRIORITY

This application claims the benefit of U.S. Provisional patentapplication Ser. No. 62/025,185, filed on Jul. 16, 2014, the benefit ofpriority of which is claimed hereby, and of which is incorporated byreference herein in its entirety.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to medical technology and inparticular aspects to systems and methods for manufacturing orthopedicprostheses. Illustratively, some aspects of the present disclosurerelate to methods for resistance welding a porous metal layer to a metalsubstrate utilizing an intermediate element.

BACKGROUND OF THE DISCLOSURE

Orthopaedic prostheses are commonly used to replace at least a portionof a patient's joint to restore or increase the use of the jointfollowing traumatic injury or deterioration due to aging, illness, ordisease, for example.

To enhance the fixation between an orthopedic prosthesis and a patient'sbone, the orthopedic prosthesis may be provided with a porous metallayer. The porous metal layer may define at least a portion of thebone-contacting surface of the prosthesis to encourage bone growthand/or soft tissue growth into the prostheses. The porous metal layermay be coupled to an underlying metal substrate.

SUMMARY

The present disclosure, in some aspects, provides apparatuses andmethods for manufacturing orthopedic prostheses. For example, suchapparatuses and methods can involve resistance welding a porous metallayer to an underlying metal substrate. The resistance welding processcan involve directing an electrical current through the porous layer andthe substrate, which dissipates as localized heat to cause softeningand/or melting of the materials, especially at points of contact alongthe interface between the porous layer and the substrate. The softenedand/or melted materials undergo metallurgical bonding at the points ofcontact between the porous layer and the substrate to fixedly secure theporous layer onto the substrate. The present inventors have realizedthat providing an intermediate element between a porous metal layer anda metal substrate can provide certain resistance welding benefits. Suchbenefits are not required of all embodiments, but they can includeproviding a more uniform pressure distribution across the weldedsurface, a preservation of the porous surface of the porous layer, andcontrol of material transfer (e.g., minimization, elimination,calculated transfer, etc.) to a porous metal of the orthopedicprosthesis, e.g., from an electrode or intermediate member.

In one particular aspect, the present disclosure provides a method ofmanufacturing an orthopedic prosthesis. One step includes providing aporous metal layer positioned against a metal substrate at an interfacebetween the porous metal layer and the metal substrate. In another step,a coated surface of an electrode is brought into contact with the porousmetal layer. In another step, an electrical current is directed to theinterface between the porous metal layer and the metal substrate to bondthe porous metal layer to the metal substrate.

In another aspect, the present disclosure provides a method ofmanufacturing an orthopedic prosthesis. One step includes providing aporous metal layer positioned against a metal substrate at an interfacebetween the porous metal layer and the metal substrate. In another step,an electrode is provided. Another step includes providing anintermediate element positioned between and in contact with theelectrode and the porous metal layer where the intermediate elementincludes tantalum in contact with tantalum of the porous metal layer. Inanother step, an electrical current is directed to the interface betweenthe porous metal layer and the metal substrate to bond the porous metallayer to the metal substrate.

In still another aspect, the present disclosure provides a method ofmanufacturing an orthopedic prosthesis. One step includes providing aporous metal layer positioned against a metal substrate at an interfacebetween the porous metal layer and the metal substrate. In another step,an electrode is provided. Another step includes providing anintermediate member positioned between and in contact with the electrodeand the porous metal layer where the intermediate member is formedseparately from the electrode and includes tantalum in contact withtantalum of the porous metal layer. In another step, an electricalcurrent is directed to the interface between the porous metal layer andthe metal substrate to bond the porous metal layer to the metalsubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsmay describe similar components in different views. Like numerals havingdifferent letter suffixes may represent different instances of similarcomponents. The drawings illustrate generally, by way of example, butnot by way of limitation, various embodiments discussed in the presentdocument.

FIG. 1 is an elevational view of a prosthetic proximal femoralcomponent, the proximal femoral component including a porous metal layercoupled to an underlying metal substrate;

FIG. 2 is a cross-sectional view of the proximal femoral component ofFIG. 1;

FIG. 3 is a front elevational view of an exemplary apparatus used toassemble the proximal femoral component of FIG. 1,

FIG. 4A is a schematic diagram of the apparatus of FIG. 3, the apparatusincluding fixtures and weld heads that are shown in an open position toreceive the proximal femoral component;

FIG. 4B is a schematic diagram similar to FIG. 4A, the fixtures and theweld heads of the apparatus shown in a closed position to hold theporous metal layer against the metal substrate of the proximal femoralcomponent;

FIG. 5 is a graphical depiction of the average bond strength betweenvarious porous layers and metal substrates in accordance with Example#1;

FIG. 6 is another graphical depiction of the average bond strengthbetween various porous layers and metal substrates in accordance withExample #2;

FIG. 7 is another graphical depiction of the bond strength betweenvarious porous layers and metal substrates in accordance with Example#3;

FIG. 8 is a graphical depiction of the tantalum concentration gradientin a diffusion bonded sample and in a resistance welded sample;

FIG. 9 is a graphical depiction of the titanium concentration gradientin a diffusion bonded sample and in a resistance welded sample;

FIG. 10 is a scanning electron microscope image taken along theinterface between a porous component and a substrate of a diffusionbonded sample;

FIG. 11 is a scanning electron microscope image taken along theinterface between a porous component and a substrate of a resistancewelded sample;

FIG. 12 is another graphical depiction of the bond strength betweenvarious porous layers and metal substrates in accordance with Example#6;

FIG. 13 is a scanning electron microscope image taken along theinterface between a porous layer and metal substrate in accordance withExample #6;

FIG. 14 is a scanning electron microscope image taken along theinterface between a porous layer and metal substrate in accordance withExample #7; and

FIG. 15 is another graphical depiction of the bond strength betweenvarious porous layers and metal substrates in accordance with Example#7.

FIG. 16 is a microscopic image of a control sample of a porous metal.

FIG. 17 is a microscopic image of the same type and grade of porousmetal as in FIG. 16 except that during a welding step the porous metalwas contacted by an electrode having a textured surface.

FIG. 18 is a microscopic image of the same type and grade of porousmetal as in FIG. 16 except that during a welding step the porous metalwas contacted by an electrode having a non-textured surface.

FIG. 19 is a perspective view of an electrode in accordance with oneexample of the present disclosure.

FIG. 20 is a schematic diagram of another exemplary welding apparatusaccording to the present disclosure.

FIG. 21 is a perspective view of an intermediate member in accordancewith one aspect of the present disclosure.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate exemplary examples of the invention and such exemplificationsare not to be construed as limiting the scope of the invention in anymanner.

DETAILED DESCRIPTION

Referring to FIGS. 1 and 2, an orthopedic prosthesis is provided in theform of a proximal femoral component 10 (e.g., a hip stem). While theorthopedic prosthesis is described and depicted herein in the form of aproximal femoral component 10, the orthopedic prosthesis may also be inthe form of any number of other orthopedic prostheses or implants, orcomponents thereof, including but not limited to a distal femoralcomponent, a distal or proximal tibial component, an acetabularcomponent, or a humeral component.

Proximal femoral component 10 of FIG. 1 includes stem 12 and neck 14,which is configured to receive a modular head (not shown). It is alsowithin the scope of the present disclosure that the head may beintegrally coupled to neck 14. In use, with stem 12 of proximal femoralcomponent 10 implanted into the intramedullary canal of a patient'sproximal femur, neck 14 and the head (not shown) of proximal femoralcomponent 10 extend medially from the patient's proximal femur toarticulate with the patient's natural acetabulum or a prostheticacetabular component. Stem 12 of proximal femoral component 10 includesan exterior, bone-contacting surface 18 that is configured to contactbone and/or soft tissue of the patient's femur.

As shown in FIG. 2, proximal femoral component 10 includes a metalsubstrate 20 and a porous metal layer 22 coupled to the underlyingsubstrate 20. Porous layer 22 may be disposed within recess 26 ofsubstrate 20. With porous layer 22 defining at least a portion ofbone-contacting surface 18, bone and/or soft tissue of the patient'sfemur may grow into porous layer 22 over time to enhance the fixation(i.e., osseointegration) between proximal femoral component 10 and thepatient's femur.

Substrate 20 of proximal femoral component 10 may comprise abiocompatible metal, such as titanium, a titanium alloy, cobaltchromium, cobalt chromium molybdenum, tantalum, or a tantalum alloy.According to an exemplary example of the present disclosure, substrate20 comprises a Ti-6Al-4V ELI alloy, such as Tivanium® which is availablefrom Zimmer, Inc., of Warsaw, Ind. Tivanium® is a registered trademarkof Zimmer, Inc.

Porous layer 22 of proximal femoral component 10 may comprise abiocompatible metal, such as titanium, a titanium alloy, cobaltchromium, cobalt chromium molybdenum, tantalum, or a tantalum alloy.Porous layer 22 may be in the form of a highly porous biomaterial, whichis useful as a bone substitute and as cell and tissue receptivematerial. It is also within the scope of the present disclosure thatporous layer 22 may be in the form of a fiber metal pad or a sinteredmetal layer, such as a Cancellous-Structured Titanium™ (CSTi™) layer,for example. CSTi™ porous layers are manufactured by Zimmer, Inc., ofWarsaw, Ind. Cancellous-Structured Titanium™ and CSTi™ are trademarks ofZimmer, Inc.

A highly porous biomaterial may have a porosity as low as 55%, 65%, or75% and as high as 80%, 85%, or 90%, or within any range defined betweenany pair of the foregoing values.

An example of such a material is a highly porous fiber metal pad.Another example of such a material is a CSTi™ layer. Another example ofsuch a material is manufactured using a selective laser sintering (SLS)or other additive manufacturing or rapid prototyping types of processes.Yet another example of such a material is produced using TrabecularMetal™ technology generally available from Zimmer, Inc., of Warsaw, Ind.Trabecular Metal™ is a trademark of Zimmer, Inc. Such a material may beformed from a reticulated vitreous carbon foam substrate which isinfiltrated and coated with a biocompatible metal, such as tantalum, bya chemical vapor deposition (“CVD”) process in the manner disclosed indetail in U.S. Pat. No. 5,282,861, the disclosure of which is expresslyincorporated herein by reference. In addition to tantalum, other metalssuch as niobium, or alloys of tantalum and niobium with one another orwith other metals may also be used.

Generally, the porous tantalum structure includes a large plurality ofligaments defining open spaces therebetween, with each ligamentgenerally including a carbon core covered by a thin film of metal suchas tantalum, for example. The open spaces between the ligaments form amatrix of continuous channels having no dead ends, such that growth ofcancellous bone through the porous tantalum structure is uninhibited.The porous tantalum may include up to 75%-85% or more void spacetherein. Thus, porous tantalum is a lightweight, strong porous structurewhich is substantially uniform and consistent in composition, andclosely resembles the structure of natural cancellous bone, therebyproviding a matrix into which cancellous bone may grow to providefixation of proximal femoral component 10 to the patient's femur.

The porous tantalum structure may be made in a variety of densities inorder to selectively tailor the structure for particular applications.In particular, as discussed in the above-incorporated U.S. Pat. No.5,282,861, the porous tantalum may be fabricated to virtually anydesired porosity and pore size, and can thus be matched with thesurrounding natural bone in order to provide an optimized matrix forbone ingrowth and mineralization.

When porous layer 22 of proximal femoral component 10 is produced usingTrabecular Metal™ technology, as discussed above, a small percentage ofsubstrate 20 may be in direct contact with the ligaments of porous layer22. For example, approximately 15%, 20%, or 25%, of the surface area ofsubstrate 20 may be in direct contact with the ligaments of porous layer22.

Referring next to FIG. 3, apparatus 100 is provided for resistancewelding porous layer 22 to substrate 20 of proximal femoral component10. Apparatus 100 is also illustrated schematically in FIGS. 4A and 4B.Apparatus 100 includes housing 110, one or more braces or fixtures 120a, 120 b, one or more weld heads 130 a, 130 b, within housing 110, eachhaving an electrode 132 a, 132 b, transformer 140, a power source orcurrent generator 150, and controller 160. Each component of apparatus100 is described further below.

Housing 110 of apparatus 100 defines an internal chamber 112 that issized to receive at least one prosthesis, such as proximal femoralcomponent 10 of FIGS. 1 and 2. According to an exemplary example of thepresent disclosure, housing 110 of apparatus 100 creates a vacuumenvironment or an inert environment in chamber 112 during the resistancewelding process. In one particular example, chamber 112 of housing 110is flushed with an inert gas (e.g., argon) and controlled to have a dewpoint less than about −60° C. and an oxygen concentration less thanabout 10 ppm.

Housing 110 may be at least partially transparent to enable a user tosee inside chamber 112. Also, housing 110 may include one or moreopenings 114 to enable a user to access chamber 112. To maintain avacuum environment or an inert environment in chamber 112, housing 110may be in the form of a glovebox. In other words, each opening 114 mayinclude a glove (not shown) or another suitable barrier that extendsinto chamber 112 to receive the user's hand while maintaining a sealaround opening 114.

Fixtures 120 a, 120 b, of apparatus 100 contact proximal femoralcomponent 10 to hold proximal femoral component 10 in place withinhousing 110 of apparatus 100. Fixtures 120 a, 120 b, may be moved apartto an open position (FIG. 4A) to receive proximal femoral component 10,and then fixtures 120 a, 120 b, may be moved together to a closed orclamped position (FIG. 4B) to hold proximal femoral component 10 inplace. It is within the scope of the present disclosure that the closedposition of fixtures 120 a, 120 b, may be adjustable to enable apparatus100 to receive and hold prostheses of different shapes and sizes.

Electrodes 132 a, 132 b, on weld heads 130 a, 130 b, of apparatus 100are connected to transformer 140 and current generator 150 via wires 152a, 152 b, respectively. As shown in FIG. 4A, each electrode 132 a, 132b, faces a corresponding side of porous layer 22. More specifically,contact surface 134 a, 134 b, of each electrode 132 a, 132 b, faces acorresponding side of porous layer 22. According to an exemplary exampleof the present disclosure, contact surface 134 a, 134 b, of eachelectrode 132 a, 132 b, is designed to substantially match the contourof the corresponding side of porous layer 22. In this example, eachelectrode 132 a, 132 b, is able to make close, even contact withproximal femoral component 10. Depending on the shape of proximalfemoral component 10, the corresponding contact surface 134 a, 134 b,may be concave, convex, or planar, for example. Such weld heads orelectrodes can be formed partially or wholly with any suitableconductive material including but not limited to copper, copper alloy,tungsten, tungsten alloy, molybdenum, molybdenum alloy, or anycombination of metals or conductors suitable as electrode material knownto those skilled in the art.

In some examples, an electrode contact surface such as contact surfaces134 a, 134 b will be particularly textured, e.g., incorporate one ormore surface features or elements, or will otherwise be configured sothat structural and/or other characteristics (e.g., surface texture,strut features, surface porosity, pore features, etc.) of a porous metalstructure contacted by the electrode during a welding step will bepreserved or substantially preserved, when such preservation is desired,after the welding step has been completed. Illustratively, an electrodesurface can be equipped with a particular microarchitecture that helpsprevent or inhibit structural changes from occurring on or within aporous metal structure during a welding step. With some designs,employing this sort of surface, as compared to using a smooth, even, orgenerally non-textured surface of the same general size, can reduce theamount of contact between the electrode and the porous metal, and inthis regard, it will be understood that the amount of contact between anelectrode and a porous metal over a given area can be set at anysuitable level. Such texturing, etc. can be incorporated into anelectrode surface upon initial formation of the electrode, or anexisting electrode surface can be modified to have such features. Inthis regard, such texturing, etc. can be imparted to an electrodesurface, for example, by cutting away, grinding away or otherwiseremoving material from an initial electrode piece to provide aparticular surface texture or other surface microarchitecture, or bywelding, adhering or otherwise adding material to an existing electrodepiece to provide a particular surface texture or other surfacemicroarchitecture, or by casting or otherwise initially forming anelectrode piece to have a particular surface texture or other surfacemicroarchitecture. In some forms, a knurled electrode surface will beutilized. Such texturing, etc. can include any number and type ofsurface features or elements including but not limited to one or moreprojections, grooves, ridges, corrugations, peaks, valleys, rings,bands, bumps, bulges, lumps, knobs, protuberances, dimples, depressions,dents, and/or other suitable projections or indentations. Such surfacefeatures or elements can be of any suitable size and shape, and they canoccur randomly along an electrode surface, or they can be arranged inregular or non-random fashion (e.g., a pattern, grid or array) along anelectrode surface or region. In some forms, an electrode surface will betextured or otherwise configured so that it somewhat mimics orapproximates one or more surface features of a porous metal structure tobe contacted by the electrode during a welding step.

Referring now to FIGS. 16-18, the first of these figures is amicroscopic image of a control sample of a porous metal, while FIG. 17is a microscopic image of the same type and grade of porous metal as inFIG. 16 except that the porous metal was contacted by an electrodehaving a textured surface (45 degree, 0.040 pitch) during a welding stepas described herein. A comparison of FIGS. 16 and 17 shows a substantialpreservation of features of the porous metal structure. FIG. 18 is amicroscopic image of the same type and grade of porous metal as in FIG.16 except that the porous metal was contacted by an electrode having anon-textured surface during a welding step as described herein. Acomparison of FIGS. 17 and 18 shows a considerable difference inpreservation of features of the porous metal structure.

With reference now to FIG. 19, shown is an illustrative electrode 200according to one aspect of the present disclosure. Electrode 200includes an electrode contact surface 201 from which a plurality ofgenerally pyramid-shaped projections 202 project. These particularpyramid-like projections, while certainly useful in certain examples ofthe present disclosure, are merely illustrative of the type, shape, etc.of projections contemplated. Projections and other suitable surfaceelements or features of any suitable shape, size and configuration canbe incorporated into an electrode surface as discussed elsewhere herein.

In addition, an electrode contact surface as disclosed herein, whetheror not textured, can incorporate an optional coating, for example, wherea coating (e.g., a metallic coating) is formed onto an outer surface ofan electrode such as electrode 132 a to provide all or part of theelectrode contact surface 134 a. Referring again to FIG. 19, in anotherillustrative example, electrode 200 can incorporate an optional coatingor attached layer 203 that provides all or substantially all of theelectrode contact surface 201. In such aspects, it will be understoodthat layered and non-layered coatings can be formed onto an electrode orother weld head component in any suitable manner for forming a coatingon a substrate including but not limited to by thermal spray, plasmaspray, electron beam deposition, laser deposition, chemical vapordeposition, dipping, and cold spray techniques. A formed layer or othercoating can be continuous or non-continuous across an electrode contactsurface. As well, a formed layer or other coating can be porous oressentially non-porous. In certain embodiments, a coating is comprisedof one or more identifiable coating layers formed onto an electrode orother weld head component where any one of these layers can have anysuitable thickness, for example, in the range of about 0.005 mm to about0.10 mm, or in the range of about 0.020 mm to about 0.20 mm, or in therange of about 0.050 mm to about 0.30 mm, or in the range of about 0.10mm to about 0.50 mm, or in the range of about 0.30 mm to about 1.0 mm.

In one illustrative example, an electrode will be formed with a firstmetallic material such as a copper alloy, and an outer surface of thiselectrode will be coated with a second metallic material such as atantalum or a tantalum alloy (e.g., using a CVD process) to provide allor part of an electrode contact surface. Such a coated electrode canthen be used in a resistance welding step as disclosed herein, forexample, where a porous metal layer (e.g., layer 22) incorporatingtantalum is being welded to an underlying metallic substrate (e.g.,substrate 20) formed with titanium or a titanium alloy. Such coatingscan reduce or eliminate the unwanted transfer of an electrode material(e.g., the copper alloy or first metallic material in the above example)to the porous metal layer during a welding step. Illustratively, theporous metal layer might be formed with tantalum or at least include itsown tantalum outer coating that is brought into contact with thetantalum coating on the electrode during a resistance welding step.Illustrative coatings for electrodes can be or include tantalum,niobium, titanium, cobalt chromium, cobalt chromium molybdenum, andcombinations and alloys thereof. While not necessary to broader aspectsof the disclosure, in some embodiments, a portion of a coating on anelectrode or other weld head component will be transferred to the porousmetal layer as a result of a resistance welding step, and in someinstances, such a transfer will be expected or even desired. In certainmethods, the amount of transfer, if any, will be controlled during oneor more welding steps.

While in some instances a coating is formed onto an outer surface of anelectrode to provide an intermediate element between the electrode and aporous metal layer (e.g., layer 22) to which a metal substrate (e.g.,substrate 20) is to be resistance welded, in some other instances, othertypes of intermediate elements are employed. For example, in somepreferred embodiments, an intermediate element includes an intermediatemember (e.g., a sheet or layer) that is formed separately from anelectrode for subsequently being positioned between the electrode and aporous metal layer (e.g., layer 22) to which a metal substrate (e.g.,substrate 20) is to be resistance welded. Such an intermediate membermay or may not be adhered or otherwise affixed or coupled to theelectrode prior to a resistance welding step. When such members are in asheet or sheet-like form, the members, or any portion thereof, can haveany suitable thickness, for example, in the range of about 0.005 mm toabout 0.10 mm, or in the range of about 0.020 mm to about 0.20 mm, or inthe range of about 0.050 mm to about 0.30 mm, or in the range of about0.10 mm to about 0.50 mm, or in the range of about 0.30 mm to about 1.0mm.

Referring now to FIG. 20, shown are intermediate members 300 a, 300 bbeing utilized in an apparatus 100′ that is otherwise similar to thatshown in FIGS. 3, 4A and 4B. In this regard, intermediate members 300 a,300 b face corresponding sides of porous layer 22. More specifically,first sides 301 a, 301 b of intermediate members 300 a, 300 b facecorresponding sides of porous layer 22. Opposite, second sides 302 a,302 b of intermediate members face corresponding electrodes 132 a, 132b. Apparatus 100′ is shown in an open type of position, for example, asmight occur during loading of the various pieces. Such an apparatus canthen be converted to a closed type of position as discussed elsewhereherein, for example, where various pieces are clamped down or otherwisemanipulated so that the intermediate members make contact with theelectrodes and the porous layer and/or other surfaces of the orthopedicconstruct. An electrical current can then be directed to one or moreinterfaces occurring between the porous layer and the metal substrateusing any of the steps, parameters or techniques disclosed herein forbonding the porous layer to the metal substrate.

According to an exemplary example of the present disclosure, first sides301 a, 301 b are made to substantially match the contour of thecorresponding sides of porous layer 22, while second sides 302 a, 302 bare made to substantially match the contour of the correspondingelectrodes prior to a welding step, although in other aspects theintermediate members are shaped as part of the resistance weldingprocess. In such examples, each intermediate member is able to makeclose, even contact with proximal femoral component 10. Depending on theshape of proximal femoral component 10, the intermediate members 300 a,300 b can include curved and/or planar features, for example, and anysuitable number of intermediate members may be utilized. Suchintermediate members can be formed to have any suitable size and shape(e.g., a sheet or sheet-like element having any suitable thickness), andthey can be formed partially or wholly with any suitable conductivematerial including but not limited to copper, copper alloy, tungsten,tungsten alloy, molybdenum, molybdenum alloy, or any combination ofmetals or conductors suitable as electrode material known to thoseskilled in the art. In addition, intermediate members such as thoseshown in FIG. 20 can be utilized as disposable elements, for example,being used as few as once or for multiple welding processes.

In some examples, sides or surfaces of intermediate members such asthose shown in FIG. 20 will be particularly textured as describedelsewhere herein, e.g., incorporating one or more surface features orelements, or will otherwise be configured so that structural and/orother characteristics (e.g., surface texture, strut features, surfaceporosity, pore features, etc.) of a porous metal structure contacted bysuch sides or surfaces of the intermediate members during a welding stepwill be preserved or substantially preserved, when such preservation isdesired, after the welding step has been completed. An illustrativeexample of one such intermediate member 400 is shown in FIG. 21. Member400 includes a first side 401 from which a plurality of generallypyramid-shaped projections 402 project. These particular pyramid-likeprojections, while certainly useful in certain examples of the presentdisclosure, are merely illustrative of the type, shape, etc. ofprojections contemplated. Projections and other suitable surfaceelements or features of any suitable shape, size and configuration canbe incorporated into an intermediate member as discussed elsewhereherein.

In addition, a side or surface of an intermediate member such as thoseshown in FIG. 20, whether or not textured, can incorporate an optionalcoating that is to come into contact with a porous metal layer (e.g.,layer 22) during a resistance welding step, for example, where a coating(e.g., a metallic coating) is formed onto a side of an intermediatemember such as first sides 301 a, 301 b. Referring again to FIG. 21, inanother illustrative example, intermediate member 400 can incorporate anoptional formed coating or attached layer 403. In such aspects, it willbe understood that layered and non-layered coatings can be formed ontoan intermediate member that is formed separately from an electrode inany suitable manner for forming a coating on a substrate including butnot limited to by thermal spray, plasma spray, electron beam deposition,laser deposition, chemical vapor deposition, dipping, and cold spraytechniques. A formed layer or other coating can be continuous ornon-continuous across an intermediate member side or surface. As well,such formed layer(s), coatings, etc. can be porous or essentiallynon-porous. In certain embodiments, a coating is comprised of one ormore identifiable coating layers formed onto an intermediate member thatis formed separately from an electrode where any one of these layers canhave any suitable thickness, for example, in the range of about 0.005 mmto about 0.10 mm, or in the range of about 0.020 mm to about 0.20 mm, orin the range of about 0.050 mm to about 0.30 mm, or in the range ofabout 0.10 mm to about 0.50 mm, or in the range of about 0.30 mm toabout 1.0 mm.

In one illustrative example, an intermediate member that is formedseparately from an electrode will be formed with a first metallicmaterial such as a copper alloy, and a first side or surface of thisintermediate member will be coated with a second metallic material suchas a tantalum or a tantalum alloy (e.g., using a CVD process), or itwill have at least one separately-formed layer of a second metallicmaterial attached thereto for coming into contact with a porous metallayer during a resistance welding step. Such coatings or attached layerscan reduce or eliminate the unwanted transfer of an intermediate membermaterial (e.g., the copper alloy or first metallic material in the aboveexample) to the porous metal layer during a welding step.Illustratively, the porous metal layer might be formed with tantalum orat least include its own tantalum outer coating that is brought intocontact with the tantalum coating or attached separately-formed layer onthe intermediate member during a resistance welding step. Illustrativecoatings or attached, separately-formed layers for attachment tointermediate members that are formed separately from an electrode can beor include tantalum, niobium, titanium, cobalt chromium, cobalt chromiummolybdenum, and combinations and alloys thereof. While not necessary tobroader aspects of the disclosure, in some embodiments, a portion of acoating or attached, separately-formed layer on an intermediate membercan be transferred to the porous metal layer as a result of a resistancewelding step, and in some instances, such a transfer will be expected oreven desired. In certain methods, the amount of transfer, if any, willbe controlled during one or more welding steps.

Referring again to FIGS. 4A and 4B, weld heads 130 a, 130 b, ofapparatus 100 may be configured to hold porous layer 22 againstsubstrate 20 during the resistance welding process. More particularly,weld heads 130 a, 130 b, may be configured to hold porous layer 22within recess 26 of substrate 20 during the resistance welding process.Like fixtures 120 a, 120 b, described above, weld heads 130 a, 130 b,may be moved away from proximal femoral component 10 to an open position(FIG. 4A) to receive proximal femoral component 10, and then weld heads130 a, 130 b, may be moved toward proximal femoral component 10 to aclosed or clamped position (FIG. 4B) to hold porous layer 22 withinrecess 26 of substrate 20. The open and/or closed positions of weldheads 130 a, 130 b, may be controlled using one or more stops 136 a, 136b, that contact corresponding flanges 138 a, 138 b, on weld heads 130 a,130 b, to limit movement of electrodes 132 a, 132 b. It is within thescope of the present disclosure that the closed position of each weldhead 130 a, 130 b, may be adjustable, such as by moving stops 136 a, 136b, to enable apparatus 100 to receive and hold prostheses of differentshapes and sizes.

Optionally, apparatus 100 may include additional braces or fixtures (notshown) that are configured to hold porous layer 22 against substrate 20during the resistance welding process. More particularly, theseadditional fixtures may be configured to hold porous layer 22 withinrecess 26 of substrate 20 during the resistance welding process.

The pressure used to hold porous layer 22 against substrate 20 ofproximal femoral component 10 during the resistance welding process maybe sufficiently low to avoid deforming or compressing porous layer 22while still resisting movement of porous layer 22 relative to substrate20. Thus, the weld pressure should not exceed the compressive yieldstrength of substrate 20 or porous layer 22. If the compressive yieldstrength of porous layer 22 is about 4,000 psi (27.6 MPa), for example,a suitable weld pressure may be as low as 100 psi (0.7 MPa), 500 psi(3.4 MPa), or 1,000 psi (6.9 MPa), and as high as 2,000 psi (13.8 MPa),2,500 psi (17.2 MPa), or 3,000 psi (20.7 MPa), or within any rangedefined between any pair of the foregoing values, for example. Porouslayer 22 may be provided in a substantially final shape before theresistance welding process to avoid having to compress or otherwiseshape porous layer 22 during the resistance welding process. As aresult, the thickness of porous layer 22 and the contact area betweenporous layer 22 and substrate 20 may remain substantially unchangedduring the resistance welding process. As discussed above, the weldpressure may be applied by weld heads 130 a, 130 b, when weld heads 130a, 130 b, are in the closed position (FIG. 4B) and/or by additionalfixtures (not shown) of apparatus 100.

Controller 160 of apparatus 100, which may be in the form of a generalpurpose computer, is coupled to transformer 140 and current generator150 to control the operation of electrodes 132 a, 132 b. Controller 160of apparatus 100 may also control the evacuation of housing 110 and/orthe flushing of housing 110 with an inert gas (e.g., argon).Additionally, controller 160 of apparatus 100 may control movement offixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, between theirrespective open positions (FIG. 4A) and closed positions (FIG. 4B).

In use, proximal femoral component 10 is loaded into housing 110 ofapparatus 100. With porous layer 22 properly disposed against substrate20 of proximal femoral component 10, controller 160 may be operated tomove fixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, from theirrespective open positions (FIG. 4A) toward their respective closedpositions (FIG. 4B). The approach pressure (i.e. the pressure at whichfixtures 120 a, 120 b, and/or weld heads 130 a, 130 b, approach proximalfemoral component 10 before making contact with proximal femoralcomponent 10) may be less than the above-described weld pressure toavoid damaging the components. For example, the approach pressure may beas low as 10 psi (0.07 MPa), 30 psi (0.2 MPa), or 50 psi (0.3 MPa), andas high as 70 psi (0.5 MPa), 90 psi (0.6 MPa), or 110 psi (0.8 MPa), orwithin any range defined between any pair of the foregoing values.

After proximal femoral component 10 is loaded into housing 110 ofapparatus 100, controller 160 may be operated to evacuate chamber 112 ofhousing 110 and/or to flush chamber 112 of housing 110 with an inert gas(e.g., argon). The vacuum or inert environment within housing 110 ofapparatus 100 may substantially prevent proximal femoral component 10from oxidizing, absorbing atmospheric contaminants, and/or becomingdiscolored during the resistance welding process.

Controller 160 may then continue moving fixtures 120 a, 120 b, and weldheads 130 a, 130 b, into their respective closed positions (FIG. 4B) tohold both porous layer 22 and substrate 20 of proximal femoral component10 in place within housing 110. As discussed above, the weld pressure(i.e., the pressure at which fixtures 120 a, 120 b, and/or weld heads130 a, 130 b, come to hold proximal femoral component 10 during theresistance welding process) may be as low as 100 psi (0.7 MPa), 500 psi(3.4 MPa), or 1,000 psi (6.9 MPa), and as high as 2,000 psi (13.8 MPa),2,500 psi (17.2 MPa), or 3,000 psi (20.7 MPa), for example.

Next, controller 160 may be operated to initiate current flow fromcurrent generator 150 to transformer 140. Current generator 150 mayoperate at a power of 4 kJ, 6 kJ, 8 kJ, 10 kJ, or more, for example.With a textured or non-textured contact surface 134 a, 134 b, of eachelectrode 132 a, 132 b, positioned against porous layer 22 of proximalfemoral component 10, the weld current flows from one electrode (e.g.,electrode 132 a via wire 152 a), through proximal femoral component 10,and out of the other electrode (e.g., electrode 132 b via wire 152 b).In an exemplary example, the source electrode 132 a, 132 b, may delivera weld current to proximal femoral component 10 as low as 20 kA, 30 kA,or 40 kA, and as high as 50 kA, 60 kA, or 70 kA, or within any rangedefined between any pair of the foregoing values, for example, toproduce weld current densities as low as 25 kA/m² (3.9 kA/cm²), 35 kA/m²(5.4 kA/cm²), or 45 kA/m² (7.0 kA/cm²), and as high as 55 kA/in² (8.5kA/cm²), 65 kA/in² (10.1 kA/cm²), 75 kA/in² (11.6 kA/cm²), or 85 kA/in²(13.2 kA/cm²), or within any range defined between any pair of theforegoing values. As the weld current flows through proximal femoralcomponent 10, controller 160 may maintain the weld pressure of fixtures120 a, 120 b, and/or weld heads 130 a, 130 b.

According to Ohm's Law (P=I²*R), the weld current (1) that flows throughporous layer 22 and substrate 20 of proximal femoral component 10dissipates as heat, with the amount of heat generated being proportionalto the resistance (R) at any point in the electrical circuit. Whendifferent materials are used to construct porous layer 22 and substrate20, the resistance (R) may be highest at the interface between porouslayer 22 and substrate 20. Therefore, a large amount of heat may begenerated locally at points of contact between porous layer 22 andsubstrate 20.

According to an exemplary example of the present disclosure, the heatgenerated is sufficient to cause softening and/or melting of thematerials used to construct porous layer 22 and/or substrate 20 which,in combination with the weld pressure used to hold porous layer 22against substrate 20, causes surface metallurgical bonding to occur atpoints of contact between porous layer 22 and substrate 20. It is alsowithin the scope of the present disclosure that metallurgical bondingmay occur at points of contact within porous layer 22. For example, ifporous layer 22 is in the form of a fiber metal pad, metallurgicalbonding may occur at points of contact between adjacent metal wireswithin the fiber metal pad.

According to another exemplary example of the present disclosure, theweld current may be delivered to proximal femoral component 10 indiscrete but rapid pulses. The weld current may be delivered to proximalfemoral component 10 with as few as 4, 6, or 8 pulses and as many as 10,12, or 14 pulses, or any value therebetween, for example. Each pulse maybe as short as 20 milliseconds, 40 milliseconds, or 60 milliseconds, andas long as 80 milliseconds, 100 milliseconds, or 120 milliseconds, orany value therebetween, for example. Between each pulse, the absence ofa weld current may promote bulk cooling of porous layer 22 and substrate20 without eliminating localized, interfacial heating of porous layer 22and substrate 20. The cooling time between each pulse may be less than 1second, and more specifically may be as short as 20 milliseconds, 40milliseconds, or 60 milliseconds, and as long as 80 milliseconds, 100milliseconds, or 120 milliseconds, or any value therebetween, forexample.

As discussed above, the weld pressure used to hold porous layer 22against substrate 20 during the resistance welding process should besufficiently low to avoid deforming porous layer 22. Due to softeningand/or melting of substrate 20 along the interface, porous layer 22 mayshift or translate slightly toward the softened substrate 20 and maybecome embedded within the softened substrate 20. Therefore, the totalthickness of proximal femoral component 10 (i.e., the combined thicknessof porous layer 22 and substrate 20) may decrease during the resistancewelding process. For example, the total thickness of proximal femoralcomponent 10 may decrease by approximately 0.1%, 0.2%, 0.3%, or more,during the resistance welding process. However, the thickness of porouslayer 22 itself should not substantially change. In other words, anymeasurable change in thickness of proximal femoral component 10 shouldresult from porous layer 22 shifting into the softened substrate 20, notfrom the compaction or deformation of porous layer 22 itself. Whenporous layer 22 is in the form of a fiber metal pad, porous layer 22 mayundergo some deformation (e.g., shrinkage) due to the formation ofmetallurgical bonds within porous layer 22. However, this deformationshould not be attributed to the weld pressure.

After delivering current to proximal femoral component 10, substrate 20and porous layer 22 will begin to cool. During this time, controller 160may be operated to maintain a forge pressure on the components. Theforge pressure (i.e. the pressure at which fixtures 120 a, 120 b, and/orweld heads 130 a, 130 b, hold proximal femoral component 10 after theweld current ceases) may be less than the above-described weld pressure.For example, the forge pressure may be as low as 40 psi (0.3 MPa), 60psi (0.4 MPa), or 80 psi (0.6 MPa) and as high as 100 psi (0.7 MPa), 120psi (0.8 MPa), or 140 psi (1.0 MPa), or within any range defined betweenany pair of the foregoing values. The forge time may be as short as 1second, 2 seconds, or 3 seconds, and as long as 4 seconds, 5 seconds, ormore, for example.

In total, the time required to resistance weld porous layer 22 tosubstrate 20 using apparatus 100 may be as short as 1 second, 10seconds, 20 seconds, or 30 seconds, and as long as 1 minute, 2 minutes,3 minutes, or more, for example. The time required may vary depending onthe thickness of porous layer 22, the current generated by currentgenerator 150, and other parameters.

Finally, controller 160 may be operated to return fixtures 120 a, 120 b,and/or weld heads 130 a, 130 b, to their respective open positions (FIG.4A). Proximal femoral component 10 may then be removed from housing 110of apparatus 100 with porous layer 22 fixedly secured to substrate 20.

Advantageously, by resistance welding porous layer 22 onto substrate 20,a strong metallurgical bond may be achieved between porous layer 22 andsubstrate 20. In certain examples, the bond strength between porouslayer 22 and substrate 20 may be at least 2,900 psi (20.0 MPa), which isthe FDA-recommended bond strength for orthopedic implants. Also, becauseresistance welding involves localized, interfacial heating of porouslayer 22 and substrate 20 and requires short cycle times, degradation ofporous layer 22 and substrate 20 may be avoided. As a result, thefatigue strength of substrate 20 and porous layer 22 may besubstantially unchanged during the resistance welding process.

Although porous layer 22 is described and depicted herein as beingbonded directly to substrate 20 of proximal femoral component 10, it isalso within the scope of the present disclosure that porous layer 22 maybe pre-bonded to an intermediate layer (not shown), which is thensubsequently bonded to substrate 20. A suitable intermediate layer mayinclude, for example, a titanium foil. Both the pre-bonding step betweenporous layer 22 and the intermediate layer and the subsequent bondingstep between the intermediate layer and substrate 20 may involveresistance welding, as described above with reference to FIGS. 3, 4A,and 4B. However, it is also within the scope of the present disclosurethat the subsequent bonding step between the intermediate layer andsubstrate 20 may involve traditional, diffusion bonding.

Examples 1. Example #1—Analysis of Trabecular Metal™ Surface Finish andThickness

A series of samples were prepared, each sample having a disc-shapedporous component produced using Trabecular Metal™ technology and adisc-shaped Tivanium® substrate. The substrates were substantially thesame, but the porous components differed in two aspects—surface finishand thickness—as set forth in Table 1 below.

TABLE 1 Interfacing Surface Finish Porous Component Group of PorousComponent Thickness (inches) 1 A Electro discharge machined (EDM) 0.060B Electro discharge machined (EDM) 0.125 C Electro discharge machined(EDM) 0.250 2 A Net shape 0.060 B Net shape 0.125 C Net shape 0.250 3 ASmeared 0.060 B Smeared 0.125 C Smeared 0.250

Before placing the interfacing surface of each porous component againstits corresponding substrate, the interfacing surface of each porouscomponent was treated as set forth in Table 1 above.

In Group 1, the interfacing surface of each porous component wassubjected to electro discharge machining (EDM), which broke off someprotruding ligaments of the porous component and leveled the interfacingsurface, making more ligaments available at the interfacing surface tocontact the underlying substrate. Therefore, EDM moderately increasedthe net contact area of the porous components in Group 1.

In Group 2, each porous component was provided in a net shape and theinterfacing surface of each porous component was not subjected tomachining after manufacturing, so the bulk porosity of the porouscomponent was retained at the interfacing surface. More specifically,the net-shaped interfacing surface was produced by coating an outersurface of a porous structure (e.g., a reticulated vitreous carbon foamstructure) with metal and then maintaining the outer, coated surfacewithout machining or shaping the outer surface. Therefore, the netcontact area of the porous components in Group 2 was retained.

In Group 3, the interfacing surface of each porous component wassubjected to physical machining to break off some ligaments of theporous component and spread out or “smear” other ligaments of the porouscomponent, which caused a substantial reduction in the surface porosityof the interfacing surface. Therefore, smearing increased the netcontact area of the porous components in Group 3.

As a result of these surface treatments, the porous components of Group2 had the least surface contact with the underlying substrate, while theporous components of Group 3 had the most surface contact with theunderlying substrate.

The samples were then assembled by resistance welding. A first quantityof power was applied to weld the 0.060 inch (1.5 mm) thick and the 0.125inch (3.2 mm) thick porous components (Groups 1A, 1B, 2A, 2B, 3A, and3B) to their corresponding substrates. A second quantity of power 50%greater than the first quantity of power was applied to weld the 0.250inch (6.4 mm) thick porous components (Groups 1C, 2C, and 3C) to theircorresponding substrates. The average bond strength for the samples ofeach Group 1-3 is depicted graphically in FIG. 5.

As shown in FIG. 5, the samples of Group 2 had higher average bondstrengths than the samples of Groups 1 or 3. Because the porouscomponents of Groups 1 and 3 had more surface contact with theunderlying substrates than the samples of Group 2, the inventors suspectthat the applied current and heat dissipated across the greater surfacecontact area, resulting in weaker bonds for the samples of Groups 1 and3 than the samples of Group 2. In contrast, because the porouscomponents of Group 2 had less surface contact with the underlyingsubstrates than the samples of Groups 1 and 3, the inventors suspectthat the applied current and heat was localized at each individualligament, resulting in stronger bonds for the samples of Group 2 thanthe samples of Groups 1 and 3.

Also, within each Group 1-3, the samples of subgroups A and C had higheraverage bond strengths than the samples of the corresponding subgroup B.For example, the samples of Groups 2A and 2C had higher average bondstrengths than the samples of Group 2B.

The decrease in bond strength from subgroup A to B within each Group 1-3may be attributed to the increased thickness of the porous componentsfrom 0.060 inch to 0.125 inch. Because the thermal conductivity oftantalum in each porous component (about 54 W/m/K) is greater than thethermal conductivity of titanium in each substrate (about 7 W/m/K), thethicker porous components of each subgroup B may act as heat sinks,conducting the heat generated at the interface away from the interfaceand into the volume of the porous component.

The increase in bond strength from subgroup B to C within each Group 1-3may be attributed to the 50% increase between the first quantity ofpower used to resistance weld the 0.060 inch thick and the 0.125 inchthick porous components and the second quantity of power used toresistance weld the 0.250 inch thick porous components. The increasedpower produces increased current flow, which results in greater heatingand a stronger bond.

2. Example #2—Analysis of Trabecular Metal™ Thickness, Weld Power, andNumber of Weld Cycles

Another series of samples were prepared, each sample having adisc-shaped porous component produced using Trabecular Metal™ technologyand a disc-shaped Tivanium® substrate. Because the net-shaped porouscomponents (Group 2) achieved the highest bond strengths in Example #1,the porous components of Example #2 were also provided in a net shape.The substrates were substantially the same, but the porous componentsdiffered in thickness. Also, the resistance welding process differed intwo aspects—power and number of weld cycles—as set forth in Table 2below.

TABLE 2 Porous Component Weld Power Number of Group Thickness (inches)(kJ) Weld Cycles 4 A 0.060 6 1 B 0.060 6 2 5 A 0.125 6 1 B 0.125 6 2 6 A0.125 9 1 B 0.125 9 2

The average bond strength for the samples of each Group 4-6 is depictedgraphically in FIG. 6. The samples of Groups 4 and 6 had higher averagebond strengths than the samples of Group 5. In fact, the samples ofGroups 4 and 6 had average bond strengths above 4,000 psi (27.6 MPa),which exceeds the FDA-recommended bond strength of 2,900 psi (20.0 MPa).

About 90.6% of the variation in bond strength may be attributed to thevaried thickness of the porous components and the varied weld power. Thenumber of weld cycles was found to be statistically insignificant.

3. Example #3—Analysis of Trabecular Metal™ Thickness and Weld Time

Another series of circular samples were prepared, each sample having adisc-shaped porous component produced using Trabecular Metal™ technologyand a disc-shaped Tivanium® substrate. The contact area between eachporous component and its underlying substrate was about 5 square inches(32.3 cm²). The substrates were substantially the same, but the porouscomponents differed in thickness. Also, the resistance welding cycletime differed, as set forth in Table 3 below.

TABLE 3 Porous Component Weld Time Group Thickness (inches)(milliseconds) 7 A 0.060 150 B 0.060 200 C 0.060 250 8 A 0.125 150 B0.125 200 C 0.125 250

After resistance welding the samples, two 1.2 inch (3.0 cm) diametertest coupons were cut from each sample for tensile testing. The bondstrength for each test coupon of Groups 7 and 8 is depicted graphicallyin FIG. 7. As shown in FIG. 7, one of the two test coupons of Group 8Bhad a bond strength greater than the FDA-recommended bond strength of2,900 psi (20.0 MPa). However, the other test coupon of Group 8B had abond strength less than 1,000 psi (6.9 MPa).

The variation in bond strength between corresponding test coupons may bedue to non-uniform pressure and/or current across each sample. Physicalexamination of the remnant material left behind after cutting the 1.2inch (3.0 cm) diameter test coupons supported the finding of varyinglevels of bond strength within each sample.

4. Example #4—Comparison Between Resistance Welding and DiffusionBonding

In addition to tensile testing, metallography was also performed tocompare the bond achieved with resistance welding to the bond achievedwith diffusion bonding.

When a porous component is diffusion bonded to an underlying substrate,atoms from the porous component and atoms from the substrateinter-diffuse. For example, when a porous component produced usingTrabecular Metal™ technology is diffusion bonded to a Tivanium®substrate, tantalum from the porous component diffuses into thesubstrate, and titanium from the substrate diffuses into the porouscomponent. The diffusion of tantalum into the substrate is showngraphically in FIG. 8, and the diffusion of titanium into the porouscomponent is shown graphically in FIG. 9. The inter-diffusion oftantalum and titanium creates a concentration gradient or aninter-diffusion layer along the interface between the porous componentand the substrate. The inter-diffusion layer between the porouscomponent and the substrate is also shown visually in FIG. 10, which isa scanning electron microscope image taken along the interface betweenthe porous component and the substrate of a diffusion bonded sample.

When a porous component is resistance welded to an underlying substrate,little or no inter-diffusion occurs. For example, the tantalumconcentration in the porous component remains substantially constant inFIG. 8, and the titanium concentration in the substrate remainssubstantially constant in FIG. 9. The lack of any significantinter-diffusion layer between the porous component and the substrate isalso shown visually in FIG. 11, which is a scanning electron microscopeimage taken along the interface between the porous component and thesubstrate of a resistance welded sample.

5. Example #5—Analysis of Weld Pressure

A series of 1 inch (2.5 cm) diameter, disc-shaped samples were prepared,the electrode interface of each sample having a surface area of about0.79 square inches (5.1 cm²). Each sample had a 0.055 inch (1.4 mm)thick porous component produced using Trabecular Metal™ technology and a0.130 inch (3.3 mm) thick Tivanium® substrate. The weld pressure wascalculated to be 4,160 psi (28.7 MPa), which was comparable to thecompressive yield strength of the porous components. As a result of thishigh weld pressure, the porous components were partially crushed duringwelding and decreased in average thickness by about 0.022 inch (0.6 mm)or 40% (from 0.055 inch (1.4 mm) to 0.033 inch (0.8 mm)).

6. Example #6—Analysis of Pulse Welding

Another series of 1 inch (2.5 cm) diameter, disc-shaped samples wereprepared, each sample having a 0.055 inch (1.4 mm) thick porouscomponent produced using Trabecular Metal™ technology and a 0.130 inch(3.3 mm) thick Tivanium® substrate. Each porous component included anEDM-shaped surface interfacing with the substrate. The resistancewelding parameters of Example #6 are set forth in Table 4 below.

TABLE 4 Weld Parameter Setting Approach Pressure 20 psi Approach Time 3seconds Weld Pressure 800 psi Forge Pressure 45 psi Forge Time 3 secondsCurrent 24 kA Current Density 30 kA/in² Controlled Atmosphere argon dewpoint < −60° C. oxygen concentration < 10 ppm

As set forth in Table 5 below, each sample received a different numberof weld current pulses, with each pulse lasting 80 milliseconds and thecooling time between each pulse lasting 80 milliseconds. Samples 1-5were prepared to evaluate up to 10 pulses. Samples 6-11 were prepared tomore specifically evaluate between 5 and 10 pulses.

TABLE 5 Sample Number of Pulses 1 1 2 4 3 6 4 8 5 10 6 5 7 6 8 7 9 8 109 11 10

A new resistance welding apparatus was designed and built to deliverthese weld current pulses in a controlled environment. The apparatusincluded the AX5000 Atmospheric Enclosure with the BMI-500 Single-ColumnGas Purification System, the KN-II Projection Weld Head with cooled,copper alloy electrodes, the IT-1400-3 Transformer, and the ISA-2000CRInverter Power Supply, all of which are available from Miyachi UnitekCorporation of Monrovia, Calif.

The overall thickness of each sample remained substantially the samebefore and after welding, indicating that the lower 800 psi (5.5 MPa)weld pressure of Example #5 successfully eliminated the distortion andcrushing of the porous component seen in Example #4 above.

Samples 1-5 were subjected to tensile testing. The tensile strength ofthe weld increased from 0 psi (0 MPa) for Sample 1 (1 pulse) to 6,882psi (47.4 MPa) for Sample 3 (6 pulses). The tensile strength of the weldremained approximately the same for Sample 4 (8 pulses) and Sample 5 (10pulses).

Samples 6-11 were then subjected to tensile testing and regressionanalysis, the results of which are presented graphically in FIG. 12. Asshown in FIG. 12, the bond strength increased with each additionalpulse. The lower 95% prediction interval intersects the 2,900 psi (20.0MPa) reference line above 9 weld pulses (see circled intersection pointin FIG. 12). Thus, at the given weld parameters, at least 10 weld pulseswould be required to consistently produce bond strengths of at least2,900 psi.

Visual inspection of the samples revealed the formation of noticeableheat-affected zones along both the bond interface (i.e., the interfacebetween the porous component and the substrate) and the electrodeinterface (i.e., the interface between the sample and the resistancewelding electrode). Due to the heat generated during the resistancewelding process, the samples may experience discoloration,bleed-through, and/or microstructure changes in such heat-affectedzones. FIG. 13, for example, depicts the heat-affected zone that formedalong the bond interface of Sample 1 above. It would be possible andwithin the scope of the present disclosure to machine away or otherwiseremove the electrode interface after the resistance welding process.However, it would not be possible to remove the bond interface after theresistance welding process without destroying the bond.

7. Example #7—Analysis of Pulsed Weld Current to Reduce Heat-AffectedZones for Net-Shaped Porous Components

To eliminate the heat-affected zones seen in Example #6 above, anotherseries of 1 inch (2.5 cm) diameter, disc-shaped samples were prepared,each sample having a 0.055 inch (1.4 mm) thick porous component producedusing Trabecular Metal™ technology and a 0.130 inch (3.3 mm) thickTivanium® substrate. Unlike Example #6, each porous component included anet-shaped, not EDM-shaped, interfacing with the substrate. Also,compared to Example #6, the samples were subjected to shorter weldpulses, longer cooling times between pulses, higher weld pressures, andhigher weld currents during resistance welding. The resistance weldingparameters of Example #7 are set forth in Table 6 below.

TABLE 6 Weld Parameter Setting Approach Pressure 20 psi Approach Time 3seconds Weld Pressure 1,000 psi Forge Pressure 62 psi Forge Time 3seconds Number of Pulses 10 Weld Time per Pulse 15 milliseconds CoolingTime 250 milliseconds Controlled Atmosphere argon dew point < −60° C.oxygen concentration < 10 ppm

As set forth in Table 7 below, the samples received pulsed weld currentsbetween 35 kA and 51 kA.

TABLE 7 Weld Current Actual Weld Current (kA) Sample Setting (kA) (forPulse 1) (for Pulses 2-10) 1a 35 33.9 34.3 1b 35 33.9 34.3 2a 39 37.538.0 2b 39 37.6 38.0 3a 43 41.0 41.7 3b 43 41.2 41.6 4a 47 43.7 45.1 4b47 44.3 45.2 5a 51 46.9 48.5 5b 51 46.9 48.5

As an initial matter, visual inspection of the samples showed that thedepth and extent of the heat-affected zones along the bond interfaceswere significantly reduced compared to Example #6 or, in some cases,were eliminated altogether. For example, Sample 1 of Example #6 (FIG.13) has a noticeably larger heat-affected zone than Sample 3b of Example#7 (FIG. 14). Some heat-affected zones remained along the electrodeinterfaces, but as discussed above, it would be possible to machine awayor otherwise remove these electrode interfaces after the resistancewelding process.

The samples were also subjected to tensile testing and regressionanalysis, the results of which are presented graphically in FIG. 15. Asshown in FIG. 15, the bond strength increased as the weld current perpulse increased. The lower 95% prediction interval intersects the 2,900psi (20.0 MPa) reference line at about 43 kA per pulse (see circledintersection point in FIG. 13). Thus, at the given weld parameters, aweld current of at least 43 kA per pulse (or a weld density of at least54 kA/in² (8.4 kA/cm²) per pulse) would be required to consistentlyproduce bond strengths of at least 2,900 psi.

Additional samples were welded at 46 kA per pulse (or a weld density ofabout 58 kA/in² (9.0 kA/cm²) per pulse) to confirm this result, but thebond strengths were inconsistent and ranged from 2,387 psi (16.5 MPa) to4,246 psi (29.3 MPa). Also, visual inspection of these additionalsamples revealed noticeable heat-affected zones along the bondinterfaces. The inventors attribute these inconsistent results, at leastpartially, to electrode wear and metal transfer onto the electrode.

8. Example #8—Analysis of Pulsed Weld Current to Reduce Heat-AffectedZones for EDM-Shaped Porous Components

Example #7 was repeated with porous components having EDM-shaped (notnet-shaped) surfaces interfacing with the substrate. None of the samplesreached a bond strength of 2,900 psi (20.0 MPa). Also, the samplesformed noticeable heat-affected zones along the bond interfaces.

9. Example #9—Analysis of Weld Pressure and Pulsed Weld Current toReduce Electrode Damage and Improve Bond Strength

To improve the results of Example #7, including reducing generalelectrode wear and metal transfer onto the electrode, Example #7 wasrepeated at a higher approach pressure of 40 psi (0.3 MPa), a higherweld pressure of 2,000 psi (13.8 MPa), and a lower forge pressure of 55psi (0.4 MPa). In accordance with Example #7, the samples were subjectedto pulsed weld currents greater than 43 kA, specifically between 45 kAand 61 kA.

At the higher weld pressure of Example #9 (2,000 psi), the inventorsnoticed less sticking between the samples and the electrodes than at thelower weld pressure of Example #7 (1,000 psi). The present inventorsbelieve that the higher weld pressures increased contact between thesamples and the electrodes, and therefore reduced the resistance and theheat generated between the samples and the electrodes.

The samples were subjected to tensile testing and regression analysis,which indicated that a weld current of at least 59 kA per pulse (or aweld density of at least 75 kA/in² (11.6 kA/cm²) per pulse) would berequired to consistently produce bond strengths of at least 2,900 psi(20.0 MPa).

Nine additional samples were welded at about 59 kA per pulse to confirmthis result, and the bond strengths of these nine additional samplesaveraged 4,932 psi (34.0 MPa), ranging from 3,174 psi (21.9 MPa) to6,688 psi (46.1 MPa). Also, visual inspection of these nine additionalsamples showed minimal presence of heat-affected zones along the bondinterfaces.

Six additional samples were welded at about 61 kA per pulse (or a welddensity of about 77 kA/in² (11.9 kA/cm²) per pulse) to further confirmthis result. Each of these additional samples had a slighter thickersubstrate, specifically a 0.170 inch (4.3 mm) thick substrate, comparedto previous tests, where the substrates were 0.130 inch thick (3.3 mm).The bond strengths of these additional samples averaged 3,968 psi (27.4MPa), ranging from 3,259 psi (22.5 MPa) to 4,503 psi (31.0 MPa). In allof these additional samples, tensile failure occurred within the porousmaterial, not along the bond interface between the porous material andthe substrate. Also, visual inspection of these samples showed nomacroscopic heat-affected zones along the bond interfaces. Furthermore,visual inspection of these samples showed microstructure changes alongthe electrode interfaces, but at very shallow, removable depths (e.g.,less than 0.020 inch (0.5 mm) in depth).

While this invention has been described as having exemplary designs, thepresent invention can be further modified within the spirit and scope ofthis disclosure. This application is therefore intended to cover anyvariations, uses, or adaptations of the invention using its generalprinciples. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

1. A method of manufacturing an orthopedic prosthesis comprising thesteps of: providing a porous metal layer positioned against a metalsubstrate at an interface between the porous metal layer and the metalsubstrate; bringing a coated surface of an electrode into contact withthe porous metal layer; and directing an electrical current to theinterface between the porous metal layer and the metal substrate to bondthe porous metal layer to the metal substrate.
 2. The method of claim 1,wherein said coated surface comprises a metallic coating.
 3. The methodof claim 1, wherein said coated surface comprises one or more materiallayers that are formed onto the electrode.
 4. The method of claim 1,wherein said bringing brings tantalum of said coated surface intocontact with tantalum of said porous metal layer.
 5. The method of claim1, wherein said coated surface includes a textured surface of theelectrode onto which at least one coating layer has been formed.
 6. Amethod of manufacturing an orthopedic prosthesis comprising the stepsof: providing a porous metal layer positioned against a metal substrateat an interface between the porous metal layer and the metal substrate;providing an electrode; providing an intermediate element positionedbetween and in contact with the electrode and the porous metal layer,wherein the intermediate element includes tantalum in contact withtantalum of the porous metal layer; and directing an electrical currentto the interface between the porous metal layer and the metal substrateto bond the porous metal layer to the metal substrate.
 7. The method ofclaim 6, wherein said intermediate element includes a coating layer thatis formed onto the electrode.
 8. The method of claim 6, wherein saidintermediate element is a coating of essentially pure tantalum that isapplied to the electrode.
 9. The method of claim 6, wherein saidintermediate element includes an intermediate member that is formedseparately from said electrode.
 10. The method of claim 9, wherein saidintermediate member includes a textured outer surface in contact withthe porous metal layer.
 11. The method of claim 9, wherein saidintermediate member is affixed to said electrode.
 12. The method ofclaim 9, wherein said intermediate member includes a copper-containingsubstrate with a tantalum-containing layer adhered to thecopper-containing substrate.
 13. The method of claim 12, wherein saidtantalum-containing layer is formed onto the copper-containingsubstrate.
 14. A method of manufacturing an orthopedic prosthesiscomprising the steps of: providing a porous metal layer positionedagainst a titanium-containing substrate at an interface between theporous metal layer and the titanium-containing substrate, providing anelectrode; providing an intermediate member positioned between and incontact with the electrode and the porous metal layer, wherein theintermediate member is formed separately from said electrode andincludes tantalum in contact with tantalum of the porous metal layer;and directing an electrical current to the interface between the porousmetal layer and the titanium-containing substrate to bond the porousmetal layer to the titanium-containing substrate.
 15. The method ofclaim 14, wherein said intermediate member is affixed to said electrode.16. The method of claim 14, wherein said intermediate member includes acopper-containing substrate.
 17. The method of claim 14, wherein saidintermediate member includes a textured outer surface in contact withthe porous metal layer.
 18. The method of claim 14, wherein saidintermediate member includes a coated surface contacting the porousmetal layer.
 19. The method of claim 18, wherein said coated surfaceincludes a metallic coating.
 20. The method of claim 14, wherein saidporous metal layer includes a net surface, formation of the net surfaceincluding: providing a porous structure including an outer surface;coating the outer surface of the porous structure with metal to producea coated outer surface; and maintaining the coated outer surface withoutmachining the coated outer surface to arrive at the net surface.